The area of technical endeavor concerned is acoustics, and more particularly the area of directed acoustics wherein sound is controlled as to level at locations polar-plottable as distances and angles relative to a sound source and a primary acoustic axis.
It is often desirable to direct sound, so that it will be loud when perceived at locations near an acoustic axis along a direction of propagation along the axis, and attenuated at other locations. As one example of many that could be cited, in public address systems it is often desirable to project sound outward from a stage or podium, but not to overwhelm persons located there, yet have the projected sound be loud enough to be heard in a back portion of an audience (which may be quite extensive and some of which may be located far from the stage). There are other situations where directed sound would be desirable, some of which may not have been universally recognized by practitioners in the art of acoustics. As an example of this other category, it has been observed by some that back-up warning devices (sometimes called back-up beepers, because of the repeating single tone on-off nature of the output of such devices almost universally adopted) are needed but very annoying to the operators of vehicles on which they are installed. Also the output from back-up beepers can be undesirable at locations outside the area into which the vehicle is backing, for example trucks backing into loading docks at night have been known to wake persons sleeping in homes a considerable distance from the loading dock and not in line with the direction of the backing.
Moreover, numerous alarms, warning signals, and the like, are intended for persons in a specific area, and it would be desirable to direct the sound emitted to that area strongly, and have the sound projected in other directions be attenuated. Examples of this situation include but are not limited to train, truck and boat warning horns, whistles and the like, emergency vehicle sirens and the like. With respect to these it can be desirable to have an option to project an acoustic warning into an area along an axis of travel of the vehicle or craft, but to not project it rearward; and thus not so loudly disturb those behind. Those behind need not hear a loud warning, because with respect to those persons the warning is not very pertinent, as the hazard is moving away from them.
Door closing hazard warning tones and alarms, systems operators' warnings and alarms, crosswalk audio annunciator tones, and the like, directed to specific persons in specific areas are additional examples. Likewise fog horns (which need not disturb those inland) and other proximity hazard warning tones can be attenuated in directions not relevant to the purpose of the warning, resulting in less noise disruption overall.
Moreover, directivity with a narrow band acoustic signal and directivity with a wide band acoustic signal can be quite different problems. While many hailing and warning acoustic signals can be advantageously directed, these are typically narrow band signals or at least typically repeat within a defined frequency range. Returning to the auditorium venue or like example given above, wideband signals such as human voice or such as music program material, which are difficult to reproduce directionally, would be advantageously directionally reproduced if there were more cost-effective ways to do that.